1. Field of Invention
This invention relates to an evaporative process for the regeneration of aqueous glycol solutions.
2. Description of Prior Art
Aqueous solutions of glycol have been used commercially and by industry for years. Glycol solutions have been used for various purposes. Two significant uses of glycol are as an antifreeze agent and as a liquid desiccant for the dehydration of gases.
In antifreeze applications, glycols are used in both open and closed systems. A typical open system use would be as a sparged de-icing fluid for external aircraft surfaces. A typical closed system use would be as a heat transfer fluid in heating or cooling loops.
Dilution or contamination of glycol by water or other materials usually reduces their performance. Reestablishment of performance requires either replacement of the glycol or treatment for the removal of the diluent water and/or other contaminants.
In open antifreeze applications, glycols are typically diluted with water originating as rain, snow, ice and sometimes seawater. In the de-icing of aircraft, dilution of the glycol is expected since the intent is to melt ice and snow from aircraft external surfaces. The drainage fluids from the aircraft during and after de-icing are diluted and contaminated glycols. These fluids are then either discarded or processed (for removal or reduction of the diluent water and other contaminants) for reuse.
Another antifreeze application of industry is in the use of glycols as pipeline additives to prevent hydrate formation. This practice is particularly common in the petroleum industry. Natural gas while in the presence of water can form solid or semi-solid hydrate barriers in pipelines and process equipment at relatively high temperatures. Glycols are injected into the pipelines to provide protection against the formation of hydrates. The difficulty imposed by this application of glycols is the treatment or disposal of the glycol laden fluids remaining after such use. Glycols are added to pipelines and process equipment only in the presence of liquid or vaporous water. This situation occurs in gathering systems, transmission lines, and equipment prior to gas treatment. Gas treatment typically includes dehydration which eliminates the potential for hydrate formation. The pipelines and equipment upstream of treatment generally are located between the production wells and the treatment facilities. In these locations a liquid commonly present with the gas is production brine. These brines contain dissolved and suspended solids. The solids impede regeneration processes because of the scaling and fouling tendencies inherent in conventional boiling regeneration for the dehydration of the injected glycols. Often, because of these difficulties, such glycol laden fluids are simply discarded, with no attempt given toward regeneration and reuse of the glycols.
Typical closed system antifreeze applications use glycols as heat transfer fluids. Contamination of these fluids does not typically result from dilution by water. Contamination generally results from suspended particulate, pipe scale, products of oxidation or reduction, water born precipitates, etc. Such contaminated glycols generally are either discarded or processed (for removal or reduction of the diluent water and other contaminants) for reuse.
Most of the glycols are relatively toxic materials not acceptable for disposal to the environment. In rare cases contaminated glycols are thermally destroyed. Generally glycols are recycled for use through various process steps such as filtration, pH neutralization, thermal regeneration and/or electrodialysis treatment. Dehydration agent glycols often only require thermal regeneration, though other process steps, such as those described in the previous sentence are commonly used.
Industrial uses of glycols as dehydration agents are widespread. For many industries glycol dehydration is the process of choice. This is particularly true for the dehydration of natural and other industrial gases. In these applications, a glycol solution is brought into direct contact with the humid or wet gas. Generally this is accomplished through direct contact equipment such as contacting towers or chambers. The glycol solution absorbs water from the wet gas through the contacting process. The resulting gas becomes dehydrated and the aqueous concentration of the glycol solution increases. The dehydration capability of the glycol decreases as the aqueous content of the glycol solution increases. In process, the high aqueous content glycol, referred to as rich glycol, is then itself dehydrated, referred to as regenerated, by the application of high temperature heat. The regenerated glycol, referred to as lean glycol, once again has an affinity for water and is reused in process.
Thermal regeneration plays a dominant role in the utilization of glycol in industry. In the regenerative process, the aqueous phase of the glycol solution is boiled off in a device typically referred to as a "reboiler. " Depending upon the level of dehydration required, glycols are sometimes further dehydrated, after treatment in a reboiler, with a dried and usually heated stripping gas. This stripping gas is used to regenerate glycols to very low water content levels.
The presence of glycol in an aqueous solution elevates the boiling temperature of the solution. Higher glycol concentrations require higher temperatures for boiling. Temperatures of between 250.degree. F. and 350.degree. F. are common operating conditions for most reboilers. To dehydrate glycols in a reboiler to the level available through the stripping process would require temperatures in excess of the thermal degradation temperature of the glycols. Final polishing, when needed, is therefore provided by the stripping process. After the stripping process the hydrated stripping gas is usually wasted or used as fuel gas for the reboiler or other process fuel requirements. Stripping is primarily a glycol polishing process to remove the small amount of water remaining after the reboiler. The majority of regeneration occurs in the thermal energy absorbing reboiler.
The thermal regenerative process for the dehydration of aqueous glycol solutions has a long history of use, though the process suffers from several disadvantages:
(a) Because of the relatively high temperature requirements, high quality energy is needed. Typically fuel oils, natural gas, coal/coke or electrical energy is utilized for operation of the reboilers. The operating costs to fuel the reboilers generally are quite high. Also the combustion byproducts from the reboilers generally contain environmentally sensitive components for which air pollution permitting and possibly emission offsetting are required. Numerous process refinements have been developed to reduce the energy requirements. Some of these refinements use methods for recapturing and reusing the energy used for the regeneration. These methods reduce the thermal energy requirements but do not reduce the temperature requirements of the thermal energy that is used. Other methods reduce the energy requirements and to a limited extent the high temperature requirements by utilizing a portion of the dehydrated gas for stripping of the glycol solution. These methods then use the rehydrated gas for combustion as a fuel gas for the reboiler component of the process. Even with the refinements, thermal regeneration of aqueous glycols require the utilization of substantial amounts of high grade thermal energy. This is disadvantageous from economic, operational and environmental standpoints. PA1 (b) The high temperature requirements of thermal regeneration forces the use of metallic materials of construction which are expensive, heavy and somewhat difficult to work with. Additionally these materials often must be corrosion resistant: thus necessitating the use of high alloys and exotic, expensive materials of construction. PA1 (c) The vapor pressure of glycols increase with temperature. At the high operating temperatures of the reboilers, a significant loss of glycol occurs. This loss results from the high vapor pressure of glycol in the reboiler and the convected loss of this glycol vapor to the environment. This loss presents both an economic and environmental penalty to the operation of the reboilers. The lost glycol must be replaced at a significant cost. The environmental effects of the glycol emissions may require emission control equipment, an expensive and often operationally unattractive requirement. Other expenses and operational problems related to environmental concerns, are the requirements for additional permitting and/or acquisition of offset emitters. PA1 (d) Glycol vapor emissions which, as described above, are significant at the high operating temperature of reboilers, confer an unpleasant odor to the local environment. This odor, if not hazardous, is certainly a nuisance for which consideration must be given. Placement and operation of the reboilers must consider the negative effects of the odor on operating personnel or other nearby human environs. PA1 (e) The reboilers of the present regenerative processes are susceptible to scale buildup and fouling of heat exchange and other surfaces. These problems result from impurities in the aqueous glycol solutions. These impurities are in the form of suspended particulates and/or dissolved materials. The suspended particulate may come from either the glycol or aqueous phase source or from the materials of construction of the process system. Dissolved materials generally come from the aqueous source of the glycol solution. Dissolved materials may also originate by dissolution of the materials comprising the process system. For example, dissolved iron is a common depositional problem in process systems where carbon steel is used as a material of construction. As the aqueous glycol solutions are heated, there can be a tendency for the contaminants to deposit and foul surfaces in the reboilers and related systems. This phenomenon is especially common at the higher temperatures which exist in reboilers. The temperature sensitivity of the deposition phenomenon generally results in the fouling of the heat exchange surfaces (the hottest surfaces) in the reboilers. Operationally this is the worst location for fouling because of the detrimental effects it has upon heat transfer efficiency. To minimize fouling and scaling problems anti-scalants, dispersents and other chemicals are commonly used in the reboilers. These materials are expensive and quite often hazardous but often are necessary for successful long term operation. PA1 (f) In order to reduce the scaling, fouling and precipitate formation problems inherent in high temperature reboilers, treatment of the aqueous glycol "upstream" of the reboiler is common. Upstream treatment equipment is typically filtration for particulate removal and occasionally electrodialysis, reverse osmosis or other methods for the removal of dissolved solids. This equipment is both expensive to acquire as well as expensive and difficult to maintain. PA1 (g) Dehydrated glycol from the reboiler is a high temperature fluid which often must be cooled prior to use for dehydration. This is accomplished either through a heat exchanger for thermal reuse in the reboiler or a separate cooling system. The required equipment adds a significant cost to the regeneration system. Also heat exchangers and/or cooling systems are vulnerable to scaling, fouling and operational difficulties. PA1 (a) Because of the low operating temperature, high grade (high temperature) heat is not required. This is a great advantage in that waste heat can be used as a thermal source. Waste heat is traditionally discarded to the environment as having no economic or operational value. The invention can utilize this wasted thermal energy for the useful purpose of regeneration of aqueous glycol solutions. The use of this thermal energy source eliminates or reduces the cost of fuel for operation of a reboiler. As a consequence the economic and operational benefits can be substantial. PA1 (b) Since waste heat can be utilized in lieu of combustion fuels, environmental emissions of fuel combustion products can be reduced or eliminated. In addition to environmental benefits, this can reduce or eliminate permitting costs, siting constraints, as well as operating difficulties, costs and scheduling. PA1 (c) Since the invention can operate at temperatures within the operational limits of inexpensive plastics, these materials can be used for fabrication of the regeneration equipment. This reduces the cost of the regeneration equipment and makes it lighter, easier to maintain and, as is often a high priority, corrosion resistant. PA1 (d) Glycol vapor pressures are dependent upon temperature. At the lower regeneration temperatures of the invention, the glycol vapor pressures are significantly reduced. As a result the glycol losses during regeneration are minimized. This is a significant economic, operational, environmental, siting and permitting advantage. PA1 (e) The low regeneration temperature of the invention and the resultant lower glycol vapor pressures result in a reduced glycol loss to the environs. The sickly sweet odor of emitted glycol is reduced. The operating and living environs are therefore made more pleasant and healthier for operating or other affected personnel. PA1 (f) The low regeneration temperature of the invention minimizes the tendency for fouling and scaling of heat exchange and other surfaces. The lower operating temperature of the invention reduces the operating difficulties produced by temperature induced scaling and fouling from contaminated aqueous glycol solutions which exhibit temperature sensitivity toward such deposition. PA1 (g) The vaporization process of the aqueous phase occurs physically separate from the heat transfer process. As a result, any precipitates which form will not foul or damage the heat transfer surfaces or process. PA1 (h) The reduced fouling and scaling tendencies at the lower regeneration temperatures minimize the requirements for pretreatment. This advantage results in lower capital and operating costs. PA1 (i) The low temperature regeneration capability minimizes heat transfer requirements for cooling of the regenerated glycol solutions. This reduces capital and operating expenses over that which would be required if high temperature regeneration were utilized. PA1 (j) The ability of the invention to use low temperature, low grade heat provides the opportunity to use waste heat as a thermal source. Typically waste heat is a byproduct of an exothermic process from which heat must be removed to facilitate process continuation.
In addition to the temperature dependent scale deposition problems inherent in the high temperature operation of reboilers, there also is a natural tendency for precipitates to form on the reboiler heat transfer surfaces as a result of aqueous phase change. Operation of the reboiler requires flashing of the aqueous phase of the solution to form a vapor. This flashing process occurs on the heat transfer surfaces of the reboiler. As the aqueous phase flashes to vapor, dissolved or suspended solids precipitate from the solution at the point of vaporization. As previously discussed, this is the worst location for fouling because of the detrimental effects it has upon heat transfer efficiency.
A continuous venting, referred to as blowdown, of the solution in the reboilers is also sometimes used in conjunction with the addition, referred to as makeup, of new cleaner fluid as an approach to minimize fouling and scaling tendencies. With this approach the requirement for the addition of new, cleaner fluids is expensive and can be operationally disadvantageous. Another disadvantage of this approach is in the problem of disposal of the blowdown. The cost for the disposal of the blowdown solution and the environmental concerns related to such disposal can be prohibitive.